A Long-term Evolution (LTE) network, as a flat network, consists of an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and an Evolved Packet Core (EPC). The E-UTRAN comprises an aggregate of Evolved NodeBs (eNBs), each of which is coupled with the EPC via an S1 interface, and the eNBs can be coupled between each other via an X2 interface, wherein the S1 interface and the X2 interface are logical interfaces. One EPC can manage one or multiple eNBs, one eNB can be under control of multiple EPCs and one eNB can manage one or multiple cells.
A Self Organizing Network (SON) is a kind of technology that can conduct network configuration and optimization automatically. The technology features self-configuration and self-optimization, and its application in the LTE enables the eNB to configure network parameters automatically according to some measurement and perform optimization according to network changes automatically, thus keeping optimum network performance and saving a lot of manpower and material resources.
For the self-optimization of handover parameters of the LTE system, parameters related to cell reselection and handover are to be optimized as per a certain algorithm based on a network operation status and handover related measurement so as to improve performance of the network. The handover herein refers to intra-system and inter-system handover of the LTE, wherein the inter-system handover refers to the handover of a Universal Mobile Telecommunications System Radio Access Network (UTRAN), a Global System for Mobile communications (GSM) or a Code Division Multiple Access (ADMA) system.
In the network, the process that the terminal carries out the handover is described as follows. A network side makes a handover decision according to signal quality of a current cell and an adjacent cell reported by the terminal based on a certain handover algorithm, and notifies the terminal to perform a specific handover flow according to the handover decision. Improper handover parameter setting may lead to ping-pong handover, handover failure or radio link failure (RLF) which are all undesired handovers and may have negative effects on user experience.
When the RLF occurs, the User Equipment (UE) performs Radio Resource Control (RRC) re-establishment. The RRC re-establishment process performed when the RLF occurs to the UE will be detailed hereinafter in conjunction with FIG. 1 to FIG. 3.
FIG. 1 is a schematic diagram of a situation that the UE is too late to hand over from Cell b to Cell a according to the related art. As shown in FIG. 1, the RLF occurs to the UE in Cell b of eNB B, or a failure occurs when the UE hands over from Cell b to Cell a, and then the UE attempts to perform the RRC re-establishment in Cell a of eNB A. The above procedure indicates that the UE is too late to hand over from Cell b to Cell a.
FIG. 2 is a schematic diagram of a situation that the UE is too early to hand over from Cell a to Cell b according to the related art. As shown in FIG. 2, the RLF occurs to the UE in Cell b soon after the UE hands over from Cell a of eNB A to Cell b of eNB B, or a failure occurs when the UE hands over from Cell a to Cell b, and then the UE selects to perform the RRC re-establishment in Cell a, namely returns to a source cell before the handover to perform the RRC re-establishment. The above procedure indicates that the UE is too early to hand over from Cell a to Cell b.
FIG. 3 is a schematic diagram of a situation that the UE selects a wrong cell when handing over from Cell c to Cell b according to the related art. As shown in FIG. 3, the RLF occurs to the UE soon after the UE hands over from Cell c of eNB C to Cell b of eNB B, or a failure occurs when the UE hands over from Cell c to Cell b, and then the UE performs the RRC re-establishment in Cell a of eNB A, which indicates that Cell b of eNB B selected by eNB C is a wrong target cell, and the right one is Cell a, namely the UE should hand over from Cell c to Cell a directly.
In the processes of the handover failure above, the cells in which the handover is initiated and the cells in which the RLF occurs keep information of the UE for the RRC re-establishment. During the RRC re-establishment procedure above, the UE carries a UE-Identity, a Cell Radio Network Temporary Identifier (C-RNTI), a short Medium Access Control Integrity protection (short MAC-I) and a Physical Cell Identifier (PCI) in an RRC re-establishment request message. The C-RNTI is allocated by a source cell (the situation of the handover failure); or the C-RNTI is allocated by a cell triggering the RRC re-establishment (other situations). The PCI is a physical address of the source cell (the situation of the handover failure); or the PCI is a physical address of the cell triggering the RRC re-establishment (other situations). The short MAC-I is obtained by adopting a KRRCint key of the source cell (the situation of the handover failure) or the cell triggering the RRC re-establishment and an integrity protection algorithm, whose inputs are the C-RNTI, the PCI and a EUTRAN Cell Global Identifier (ECGI). It shall be noted that the C-RNTI and the PCI are carried by the RRC re-establishment request message above, and the ECGI is a cell identifier of a target cell selected by the UE during the RRC re-establishment.
Furthermore, when the UE performs the RRC re-establishment procedure, if the RRC re-establishment is unsuccessful, that is, the RRC re-establishment fails, then the UE switches into an idle (IDLE) state and waits to reselect another cell to perform the RRC establishment.
FIG. 4 is a schematic diagram of the processing of an RRC re-establishment failure according to the related art. As shown in FIG. 4, the RLF occurs to the UE in Cell 2, and then the UE attempts to perform the RRC re-establishment in Cell 1 and switches into the IDLE state after the attempt of performing the RRC re-establishment fails, at last the UE reselects Cell 3 and succeeds in performing the RRC establishment.
A judgment method for handover situations is that: after the RRC establishment succeeds, the UE is required to report one or multiple items of a cell global identifier of a cell where the RLF occurs to the UE, a cell global identifier of a cell where the first RRC re-establishment is attempted, a global identifier of a serving cell of the UE before the RLF occurs (optional), a time difference between the last handover and the first re-establishment attempt of the UE, RLF measurement information, etc., wherein the cell identifier herein refers to the ECGI of a cell. The base station 3, to which Cell 3 belongs, sends the related information above reported by the UE to the base station 2, to which Cell 2 where the RLF occurs to the UE belongs, and the base station 2 judges what kind of handover situation occurs based on the information, so as to conduct optimization. For example, the judgment method that can be adopted is described as follows. If the time difference reported by the UE does not exist or is greater than a set threshold, for instance, the threshold can be the time period of a timer for storing the UE context, it can be judged to be a situation of too late handover (Too Late HO). If the time difference reported by the UE is less than the set threshold and the cell where the UE conducts the first RRC re-establishment is the source cell of the previous handover of the UE, it can be judged to be a situation of too early handover (Too Early HO). If the time difference reported by the UE is less than the set threshold and the cell where the UE conducts the first RRC re-establishment is not the source cell or the target cell of the previous handover of the UE or the serving cell where the RLF occurs to the UE, it is judged to be a situation of handover to a wrong cell (HO to a Wrong Cell). For the failure during the handover, if the judgment method above is adopted, the base station 1 to which the cell (Cell 1), where the handover is initiated, belongs shall attempt to acquire the cell global identifier of the target cell (Cell 2) of the handover and RLF-related measurement information of the cell (Cell 2) where the RLF occurs. The RLF-related measurement information comprises measurement results of the signal quality of a best surrounding cell and a serving cell before the RLF occurs, wherein the measurement results of the signal quality are recorded by the UE. The RLF-related measurement information can also comprise measurement results of the signal quality of the serving cell and the best surrounding cell measured by the UE when the UE initiates the re-establishment subsequently, and location information of the UE.
However, the inventor found that the UE can not acquire the cell global identifier (which is ECGI in the E-UTRAN) of the target cell (Cell 2) of the handover when failure occurs during the handover procedure, which causes the base station 1 to which the cell (Cell 1), where the handover is initiated, belongs can not acquire the cell global identifier (which is ECGI in the E-UTRAN) of the cell where the RLF occurs, further causes the cell (Cell 1) where the handover is initiated can not carry accurate judgment of handover situations, and the optimization of handover parameters.